I've just noticed that when I log in on a test account which has German set in the user cp (forum default language is English) that I get no advanced search results returned for some terms which do indeed return results in the default language. I'm guessing that this might be happening in other languages too.
This happens on my own account too. I just used advanced search to look for a word (Bannführer) and got results using the forum default language of English. I then switched the language to German (by clicking on the flag under my navbar), performed the same search and got no results for the same word. Any ideas?